A basic element of a solid electrolytic capacitor is generally produced, as shown in FIG. 1, by forming an oxide film layer 2 as a dielectric material on an anode substrate 1 comprising a metal foil having a specific surface area enlarged through etching treatment, forming a solid semiconductor layer (hereinafter referred to as a solid electrolyte) 4 as an opposing electrode in the outer side of the oxide film layer, and if desired, further forming an electrically conducting layer 5 such as electrically conducting paste. To an element thus obtained or elements stacked, lead wires 6 and 7 are joined and the entire is completely molded with an epoxy resin 8 or the like. The thus-produced element is being widely used as a capacitor 9 component for electrical products.
With recent progress on digitization of electric equipment and speeding-up of PC operation, a small-size large-capacitance capacitor and a capacitor having a low impedance in the high frequency region are demanded. Recently, use of an electrically conducting polymer having electron conductivity as a solid electrolyte has been proposed.
As for the shape of solid electrolyte, a technique in which, by welding a metal to an aluminum foil, an electrolytically polymerized conductive polymer is formed all over the surface of an aluminum foil with the metal being as the starting part of polymer growth through electrolytic oxidation has been proposed (see, for example, JP-A-4-307917, (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)).
Also, for the purpose of enhancing electrostatic capacitance or reducing the size, aggressive development on technique of enlarging the effective surface area of the anode foil is being made. For example, a technique of etching an aluminum foil and then rolling the etched layer has been proposed (see, for example, Japanese Patent Number 3084330 and JP-A-14-260968).
Furthermore, a small-size and high-density semiconductor package is demanded, and a thin and substrate-contacting solid electrolytic capacitor which is incorporated into a substrate has been proposed (see, for example, JP-A-14-260967).